1. Field of the Invention
The present invention relates to a battery device including a charge/discharge control device configured to control the charging and discharging of a secondary battery.
2. Description of the Related Art
A battery device of the related art is illustrated in a circuit diagram of FIG. 3.
The battery device of the related art includes a secondary battery 11, a charge/discharge control device 24, an external positive terminal 25, and an external negative terminal 26. The charge/discharge control device 24 includes a charge/discharge control circuit 14, resistors 12 and 15, a capacitor 13, and an N-channel (Nch) charge/discharge control switch 23. The charge/discharge control circuit 14 includes a positive power supply terminal 18, a negative power supply terminal 19, a discharge control signal output terminal 20, a charge control signal output terminal 21, and an overcurrent detection terminal 22. The Nch charge/discharge control switch 23 includes an Nch discharge control field effect transistor 16, and an Nch charge control field effect transistor 17.
The charge/discharge control circuit 14 is configured to monitor the voltage of the secondary battery 11 with a voltage that is applied between the positive power supply terminal 18 and the negative power supply terminal 19. When the voltage of the secondary battery 11 exceeds an overcharge detection voltage, the charge/discharge control circuit 14 outputs a charge prohibiting signal from the charge control signal output terminal 21 to turn the Nch charge control field effect transistor 17 off, to thereby stop charging. This is called an overcharge detection function. When the battery voltage of the secondary battery 11 drops below an overdischarge detection voltage, the charge/discharge control circuit 14 outputs a discharge stopping signal from the discharge control signal output terminal 20 to turn the Nch discharge control field effect transistor 16 off, to thereby stop discharging. This is called an overdischarge detection function.
The charge/discharge control circuit 14 is also configured to monitor a current flowing through the battery device with the voltage of the overcurrent detection terminal 22. When the voltage of the overcurrent detection terminal 22 exceeds a discharge overcurrent detection voltage, the charge/discharge control circuit 14 outputs a discharge stopping signal from the discharge control signal output terminal 20 to turn the Nch discharge control field effect transistor 16 off, to thereby stop discharging. This is called a discharge overcurrent detection function. When the voltage of the overcurrent detection terminal 22 drops below a charge overcurrent detection voltage, the charge/discharge control circuit 14 outputs a charge prohibiting signal from the charge control signal output terminal 21 to turn the Nch charge control field effect transistor 17 off, to thereby stop charging. This is called a charge overcurrent detection function.
Currently, battery devices that include two charge/discharge control devices are often used as safer battery devices. Providing a battery device with two charge/discharge control devices means that, in the event of a failure of a first charge/discharge control device to operate, a second charge/discharge control device comes into operation, and hence safety of the battery device can be enhanced.
FIG. 4 is a circuit diagram for illustrating a battery device with enhanced safety.
The battery device of FIG. 4 includes a secondary battery 11, two charge/discharge control devices 24, an external positive terminal 25, and an external negative terminal 26. Providing the battery device with the two charge/discharge control devices means that, in the event of a failure of the first charge/discharge control device 24 to operate, the second charge/discharge control device 26 comes into operation, and hence safety of the battery device can be enhanced.
However, when an overcurrent detection function with good detection accuracy is to be realized in the related art as illustrated in FIG. 4, the following problem arises.
The charge/discharge control circuit 14 is configured to monitor a current flowing through the battery device with a voltage between the negative power supply terminal 19 and the overcurrent detection terminal 22. The accuracy of the voltage is determined based on the accuracy of ON resistances of the Nch discharge control field effect transistor 16 and the Nch charge control field effect transistor 17. In general, manufacturing variation of ON resistances of field effect transistors is large. Thus, there is a problem in that it is necessary to make a selection of field effect transistors to use only four field effect transistors having desired ON resistances, and the manufacturing cost is increased as a result.